Had a couple questions whether it's useful to buy energy.First upgrade is at Level 5, next is at level 10.

Short answer: Yes. It's good to buy Energy when you can.Long answer:Each of these increases gives you an extra 10 energy. So, if you do not log in all the time, that 10 energy is stored when it would otherwise be wasted. So assuming you log in twice per day, infrequently, 10 extra energy will give you 30 extra energy over the course of a day.That 30 extra energy will translate into 300 gold per day, given mission 45. It's actually more, as you get bonus money when you complete a mission. So 10k to buy it, it will make itself back in a few weeks. It'll allow you to do an extra 2-3 missions a day, hopefully, so you can get through the missions faster.

However. If you're having trouble beating missions and need cards, then likely it's better to buy a gold pack and put off buying Energy. If you're not having trouble beating missions, buy the energy.